METHICILLIN RESISTANCE IN A METHICILLIN SENSITIVE STAPHYLOCOCCUS AUREUS: THE ROLE OF BIOFILM IN RESISTANCE MECHANISMS OF CELL WALL DEFICIENT ORGANISMS

Abstract

A phenomenon of methicillin resistance in methicillin sensitive Staphylococcus aureus has been noted in organisms living in biofilm induced by the state of cell wall deficiency. The rate and the amount of biofilm formed by the cell wall deficient organisms far exceeds that of cell wall patent organisms. Once removed from the biofilm the S. aureus had the same sensitivities of the original organism. Cell wall deficient organisms outside the biofilm did not demonstrate the methicillin resistance. A known laboratory strain (ATCC 9144) was induced into a cell wall deficient state and allowed to form biofilm. The rate of formation and amount formed was compared with that formed by cell wall patent organisms. Before inducing cell wall deficiency sensitivity to methicillin was demonstrated using standard microbiological technique. Using an oxacillin containing plate as a culture medium: the biofilm, cell wall deficient organisms and the cell wall competent organisms were inoculated onto separate media. Organisms from the biofilm were isolated and grown free of the biofilm on blood agar. Any growth on the oxacillin containing plate would demonstrate methicillin resistance. There was no growth on the plates containing the cell wall competent or cell wall deficient organisms. There was however growth on the plate inoculated with bio-film, however when organisms were isolated from the biofilm, there was no growth on the media. Antibiotic sensitivities of the original inoculant and the organisms isolated from the biofilm were the same. The biofilm, induced as a result of cell wall deficiency, offers a form of structural protection to the Staphylococcus aureus without altering the resistance pattern of organism. Standard microbiological techniques would therefore report the organism as methicillin sensitive, however clinically the organism may behave as a methicillin resistant organism. The state of cell wall deficiency encourages the formation of biofilm in S. aureus. In-vitro the state of cell wall deficiency is induced using high osmolality media or sub-lethal doses of cell wall active antibiotics. Both these states are found in clinical practice.